diff --git a/rx-preferences/src/main/java/com/f2prateek/rx/preferences2/StringSetAdapter.java b/rx-preferences/src/main/java/com/f2prateek/rx/preferences2/StringSetAdapter.java
index 281c1d8..ae0270c 100644
--- a/rx-preferences/src/main/java/com/f2prateek/rx/preferences2/StringSetAdapter.java
+++ b/rx-preferences/src/main/java/com/f2prateek/rx/preferences2/StringSetAdapter.java
@@ -3,6 +3,7 @@
 import android.annotation.TargetApi;
 import android.content.SharedPreferences;
 import android.support.annotation.NonNull;
+import java.util.Collections;
 import java.util.Set;
 
 import static android.os.Build.VERSION_CODES.HONEYCOMB;
@@ -12,7 +13,7 @@ final class StringSetAdapter implements Preference.Adapter<Set<String>> {
   static final StringSetAdapter INSTANCE = new StringSetAdapter();
 
   @Override public Set<String> get(@NonNull String key, @NonNull SharedPreferences preferences) {
-    return preferences.getStringSet(key, null);
+    return Collections.unmodifiableSet(preferences.getStringSet(key, null));
   }
 
   @Override public void set(@NonNull String key, @NonNull Set<String> value,